1. Field of the Invention
This invention pertains generally to the field of woodworking, and more specifically to processes of restoring a stripped screw hole in a piece of finished dressed lumber.
2. Description of the Related Art
Many household products and many building structures are manufactured from wood and wood products, including such items as cabinets, doors, windows, furniture, and other items too numerous to list. Wood has many desirable characteristics which enables application in such diverse products. Unfortunately, joints and connections have traditionally failed long before the wood requires replacement.
These joints and connections frequently involve the use of a screw or other similar fastener, since such fastener is removable. The use of a removable fastener confers several benefits, including the ability to remove or replace the attached article. For example, doors such as may be used for human passage or providing closure for enclosed spaces may periodically become worn, damaged or otherwise require repair and replacement. Further, over time, such portals may also require minor re-adjustment.
Unfortunately, the same factors that may result in the wear or damage to the door may also inadvertently result in the damage of the screw hole within which the screw is anchored. Such damage has been relatively difficult to fix in the prior art, and persons have resorted to inserting tooth picks, steel wool or other materials into the hole together with the fastener to provide a pressure or frictional force fit to restore the hole. As is well known, none of these techniques provide the desired long-term strength that was present in the original hole, nor will these types of repairs be close in initial peak strength.
Some more effective, though more complex, repair techniques have long been used, including the drilling of a cylindrical hole followed by the insertion of a dowel or similar wooden pin therein. Several exemplary U.S. patents illustrate this technique, including U.S. Pat. No. 276,499 by Story and U.S. Pat. No. 5,052,333 by Woudsma, each which are incorporated herein by reference for their teachings. The dowel and hole must both be carefully prepared for this repair technique to be effective. Unfortunately, in the case of a stripped hole in wood, the hole will all too frequently be out-of-round, and may also be off-center from the original hole location. In other words, as the screw strips the hole, the hole will frequently be enlarged unevenly, such as with a bias in one direction or another. Consequently, the use of a standard drill bit to drill a standard cylindrical hole of slightly larger diameter is difficult, since the alignment of the larger drill bit may be difficult or impossible, subject to the geometry of the stripped hole. Even with a suitable geometry to accommodate drilling a cylindrical hole, the depth of the hole is both critical and also very difficult to judge. If the drilled hole is too shallow, the cylindrical repair plug will protrude from the hole, requiring cutting or sanding which can be both difficult and tedious, particularly where the surrounding wood would preferably not be altered or damaged in any way. If the hole is too deep, the plug may pass into the hole well below the surface of the surrounding wood, and in the process weaken the holding power of the newly inserted screw, owing to the reduced amount of wood for the screw to anchor into.
The cylindrical plug suffers from a second drawback. The holding power of the plug is frequently somewhat marginal, since the plug and/or the hole may not be of identical geometry. This may occur when one or the other are not quite cylindrical, or where the plug and hole diameters do not match well. In either case, the surface area for bonding is reduced, and the shear strength between the plug and surrounding material becomes much more dependent upon the shear strength of the adhesive. This is not desirable, but is a frequent result of the typical not-quite-cylindrical hole and plug preparation.
In order to reduce the effect of mismatched holes, some have proposed the use of a conical plug, such as U.S. Pat. No. 4,295,763 by Cunniff, the teachings which are incorporated herein by reference. The use of a slightly conical plug permits the plug to have a small region with diameter slightly larger than a hole, thereby inducing elastic deformation between plug and hole, and likewise compensating for any mismatch in diameter between plug and hole. Unfortunately, such technique does not provide a large surface area for the bonding, since the only region of contact is usually adjacent the outer surface of the repaired wood where the plug has the largest diameter. Farther in, the conical plug reduces in diameter below the diameter of the cylindrical hole and thereby forms a gap between plug and hole, making this conical plug and cylindrical hole poorly adhered relative to hole depth.
Others have proposed the use of a tapered bore and tapered repair plug. An exemplary patent, the teachings of which are incorporated herein by reference, is U.S. Pat. No. 4,894,971 to Cortese. Unfortunately, in the Cortese patent holes of a variety of shapes are introduced, several which would appear impossible to form or produce. The relevant tapered plug is tapered from large at both surfaces of the wood to a minimum at the center, which requires access at both surfaces of the wood, a luxury which is only extremely infrequently available. Furthermore, in the Cortese concept, the holes and plugs must still be fitted very precisely, or the opposed plug portions will mate together but not to the surrounding wood, or alternatively will mate to the surrounding wood and not to each other, which would in either case result in an inferior repair.
Since the advent of suitable liquid adhesives, semi-liquid resins, pastes and the like, a technique has been used which involves the introduction of resin into the damaged hole, followed by a suitable cure period, and then a drilling of appropriate pilot hole. This technique is also widely used, but requires substantial time for the hardening of the filler material. Further, the filler material does not have the intrinsic strength or appearance present in the natural material. Finally, the expansion coefficients due to moisture and temperature, and also the aging and discoloration over time are each frequently very different from wood, resulting in a tendency for this type of repair, even when perfectly executed, to degrade much more rapidly than a wood plug over time.
Each of the foregoing techniques suffer from yet another drawback. Once the stripped hole is appropriately filled, introduction of a screw or similar fastener is extremely difficult or impossible without the drilling of a pilot hole. Unfortunately, there is no indication or guide left for the proper alignment of the new pilot hole. Consequently, it is entirely possible, and all to commonplace, for the pilot hole to be inadvertently placed or drilled at an improper angle or an off-center position relative to the original fastener location.